The invention relates to a process and apparatus for coating printed circuit boards with a coating that is crosslinkable by electromagnetic radiation, especially UV radiation
The function of the printed circuit board is to provide the conductive connection to the components. As miniaturisation progresses, the number of connections is becoming ever greater, with the result not only that printed circuit board technology has produced multilevel circuitry, but also that conductive tracks are becoming ever narrower, drill hole diameters are becoming ever smaller and the number of conductive tracks between two holes is becoming ever greater.
With the development of surface-mounted devices it has been possible to achieve a further reduction in the surface area required. This has led to conductor widths of less than 100 .mu.m, to drill hole diameters of from 0.3 to 0.2 mm and to solder point diameters of only up to 0.4 mm with up to seven conductive tracks between a drill hole grid of 2.54 mm. At the same time, more and more connections must be made per integrated circuit, which results in connection pad grids of from 0.3 to 0.4 mm. The problems resulting from the increasingly high integration density are very complex and require a comprehensive solution. They begin first with the production of the conductive pattern.
For the production of a conductive pattern, the drilled copper-clad base material is coated with a positive or negative resist. While such resists were in the past screen-printable etching or electro resists, nowadays photoresists are predominantly used, which are solid or liquid resists. They are either laminated on to the surface (solid resists) or applied by means of a pouring machine or using rolls (liquid resists). After the application of a mask, the conductive pattern is fixed, for example, by exposure to UV light, with polymerisation of the resist, and exposed by development. This leads to the so-called pattern plating process. In pattern plating, first of all a mask is applied and only the conductor-free areas are exposed and developed. The conductive tracks are then built up by electroplating and the drilled contact holes are clad with copper. After the conductors have been deposited by electroplating, they are, for example, provided with a tin coating, the electro resist is removed and the conductive pattern is etched. Since the etching speed is the same in all directions, the undercutting corresponds approximately to the thickness of the copper film used. The undercutting that occurs in the etching stage of the pattern plating process represents the limit for large-scale application of that process. Moreover, the production of conductors of equal height is frequently not possible as a result of the geometry of the bath or of the printed circuit board.
Accordingly, for microconductor technology, the so-called panel plating process was developed. In that process, starting with the drilled copper-clad base material, first of all the surface of the printed circuit board and the drilled holes are clad with copper by electroplating in order to achieve a uniform thickness of the copper layer. A dry film resist is then laminated on, exposure is effected using a mask, and development is carried out.
In the case of microconductors, however, constrictions frequently occur because the line pressure of the laminating roller is unable to compensate for uneven areas of the base material, so that the dry film resist does not adhere to the same extent in all places. It is especially important that the already copper-clad drill holes should also be protected from the effects of the etching. That is achieved by covering the holes with resist, so-called "tenting".
Further miniaturisation and the technology of surface-mounted components has led to so-called "rest-ring"-free through-holes. Dry film lamination technology cannot be used in this case, because without the so-called rest-rings the resist film cannot be attached to the surface of the printed circuit board. However, in order to be able to cover also rest-ring-free holes with resist and thus protect them from the effects of the etching, so-called electro-immersion coating was found, which deposits a resist film from 5 to 15 .mu.m thick from a coating bath in the hole and on the surface of the printed circuit board. However, that process is very cost-intensive and, because of the thinness of the coating, can be used only together with the panel plating process.
WO 93/14444 proposes a hot coating process which is based on the use of a meltable photoresist that is highly viscous at room temperature, which photoresist, after liquefaction, is coated on to cooled printed circuit boards using the curtain pouring process. As the photomelt resist comes into contact with the cooled printed circuit boards, it is cooled at the walls of the drill holes, so that it is unable to run into the drill holes and forms over the hole a covering supported at the wall of the hole. Although cooling the printed circuit boards produces good results with printed circuit boards that are still unstructured, the technique cannot be used for cooled printed circuit boards. In that case, the conductive tracks act as cooling fins. The photomelt resist cools suddenly as it comes into contact with the conductive surface, as a result of which air can become trapped between the conductive tracks.
There are also known processes in which a coating composition is coated on to printed circuit boards by means of steel rolls. The simultaneous coating of both sides of the printed circuit boards in a roll coating apparatus having two heated applicator rolls is especially economical. For that purpose it is customary to use surface-coating systems that contain a high proportion of solvent and have a low viscosity. The surface tension must be low so that the coating is able to spread easily without forming streaks. This two-sided coating process requires the surfaces of the printed circuit boards to be as even as possible. In the coating of printed circuit boards that are already provided with conductive tracks, constrictions may occur over the conductors, so that reliable covering is not guaranteed. In addition, the hollow spaces that are present as a result of undercutting beneath the edges of the conductors are frequently not filled with coating, which leads to defective areas after soldering and allows moisture to penetrate.
Accordingly, the problem underlying the present invention is to provide a coating process that permits the production of coatings that are free of air bubbles. In particular, the process is to permit the simultaneous coating of both sides of printed circuit boards. It is also to be possible to coat printed circuit boards having relatively high conductive tracks and at the same time reliably to cover any through-plating holes that are present. Undercuts at the edges of the conductors are to be filled completely with coating. After drying, the surface of the coating is to be substantially non-tacky and permit exposure using the contact-exposure process. There is also to be provided an apparatus that allows the process according to the invention to be carried out.